1,1-Dimethyl-1-silacyclobutane was polymerized anionically with sec-butyllithium as an initiator in various solutions. In n-hexane solution with N,N,N',N'-tetramethyl ethylenediamine, high molecular weight poly(1,1-dimethyl silabutane) was prepared without precipitation during propagation. After completion of the polymerization, the chain length of the polymer was extended stoichiometrically by successive additions of the monomer or tert-butoxy styrene to the polymer solution. The regularity of the head-to-tail configuration for the polymer was analyzed by H-1 NMR, C-13 NMR, and two-dimensional H-1-C-13 correlation measurements. Differential scanning calorimetry and X-ray diffraction measurement demonstrated that poly(1,1-dimethyl silabutane) is a crystalline polymer.